The present invention relates generally to touchscreens and, more particularly, to a method and apparatus for discriminating between a false touch event and a true touch on a touchscreen.
Touchscreens are used in conjunction with a variety of display types, including cathode ray tubes (i.e., CRTs) and liquid crystal display screens (i.e., LCD screens), as a means of inputting information into a data processing system. When placed over a display or integrated into a display, the touchscreen allows a user to select a displayed icon or element by touching the screen in a location corresponding to the desired icon or element. Touchscreens have become common place in a variety of different applications including, for example, point-of-sale systems, information kiosks, automated teller machines (i.e., ATMs), data entry systems, etc.
A variety of touchscreen types have been developed. Unfortunately each type of touchscreen has at least one weakness limiting its usefulness in at least some applications. For example, the cover sheet in a resistive touchscreen is susceptible to damage. Even repeated screen compressions may eventually damage a resistive touchscreen. This type of touchscreen is also susceptible to environmental damage, for example moisture entering the display. A second type of touchscreen, capacitive touchscreens, are non-responsive to touch from an ungrounded object, thus leading to potential problems with gloved hands, styluses, pencils, etc. A third type of touchscreen utilizing surface acoustic waves is susceptible to the accumulation of contaminants (e.g., water) on the surface of the sensor. Contamination can also interfere with the operation of infrared touchscreens. A fifth type of touchscreen using force sensors is susceptible to shock and vibration.
Various systems have been designed that utilize two different touchscreen technologies for a variety of purposes, primarily as a means of accommodating different touch mechanisms, e.g., a finger and a stylus, for data entry.
U.S. Pat. No. 5,231,381 discloses a multi-purpose data input device utilizing an integrated touchscreen and a digitizing tablet. The touchscreen detects the presence and location of a passive input (e.g., finger touch) through any of a variety of techniques including surface acoustic wave, force, capacitive, or optical touch sensors. The digitizing tablet employs an active stylus mechanism to stimulate a capacitive, inductive, or surface acoustic wave sensor.
U.S. Pat. No. 5,510,813 discloses a touch panel that measures both touch position and touch force. The touch panel uses a resistive, conductive layer and determines touch position by monitoring the current pattern. The force of the touch is determined by monitoring a capacitance value between the touch panel and a second conductive panel that extends substantially parallel to the touch panel. In response to a touch, the system processes both the detected position and the detected force of the touch.
U.S. Pat. No. 5,543,589 discloses a dual sensor touchscreen in which each sensor determines touch position, but with a different resolution. The two sensors are sandwiched together to form a single sensor, thus allowing a single touch by a finger, stylus, etc. to be detected by both sensors. In use, the wide conductors of the low resolution sensor are first scanned in order to determine touch position to within a rectangular area the size of one wide conductor. To determine the touch location with the higher resolution sensor, only the narrow conductors corresponding to the rectangular area of touch determined with the low resolution sensor must be scanned. Thus the system disclosed is intended to reduce the number of scan drivers and receivers required, thus lowering cost as well as speeding up the scanning process.
U.S. Pat. No. 5,670,755 discloses a touch panel that can be used in either of two modes. In one mode, the touch panel operates like a conventional touchscreen, allowing a user to input information by touching the screen with a finger, pen, or other touching medium. In this mode two resistive layers applied to the panel come into contact at the point of touch. The determination of the contact location is based on resistance ratios. In a second mode, the touch panel functions as a digitizer using a specially designed stylus. Capacitance coupling at the contact point of the stylus to the panel is used in determining the contact point.
U.S. Pat. No. 5,777,607 discloses a system that senses finger touch capacitively and stylus touch resistively. In either touch mode the disclosed system is able to determine the x- and y-coordinates of the touch on the touchscreen using a single resistive layer. In the preferred embodiment, the finger detection mode is disabled when the system detects the stylus is in use, thus preventing the inadvertent input of data through capacitive coupling with the user""s hand.
U.S. Pat. No. 5,801,682 discloses a dual sensor touchscreen in which the variations in coordinate data from a capacitive sensor are compensated for by the use of strain gauges mounted at the corners of the sensor. Variations in the capacitive sensor data may result from changes in signal path, for example, due to the user wearing gloves.
What is needed in the art is a method and apparatus for discriminating against false touches of the sort that may result from external stimuli such as vibration, electrical noise, and contaminants, or for confirming the presence of touch. The present invention provides such a method and apparatus.
The present invention provides a method and apparatus for discriminating against false touches in a touchscreen system. The system utilizes multiple touchscreen sensors of differing types to validate a touch on a touchscreen. Thus the invention utilizes the strengths of specific sensor types to overcome the deficiencies of other sensor types.
The basis of the invention lies in the ability to confirm a touch registered by one touch sensor with another touch sensor. If the touch is confirmed, the touch can be acted upon, for example by sending touch coordinates to the operating system. If, on the other hand, the touch is not confirmed, the touch is invalidated. The system can be designed such that there is a primary touch sensor that determines the touch coordinates and a secondary sensor that validates the presence of a touch, by either a discrete signal or by generating a second set of touch coordinates for comparison purposes. Furthermore, the touch coordinates can either be determined before or after the initial touch is confirmed.
In one embodiment of the invention, the secondary touch sensor comprises a force sensor to discriminate between true and false touches, false touches being caused by such factors as contaminants (e.g., problematic for optical and surface acoustic wave sensors), noise or weak signals (e.g., problematic for capacitive sensors), etc. In this embodiment, when the pressure that is applied to the touch screen exceeds a predetermined threshold, the touch coordinates are validated. In contrast, when a false touch signal is generated by the primary sensor, for example due to a raindrop falling on a surface acoustic wave sensor, the lack of a confirming touch pressure on the secondary force sensor causes the data to be declared invalid. The secondary force sensor may be a simple one-element system that merely indicates that a touch has occurred by sensing touch pressure, or a multi-element force touch system that can provide confirming or supplementary coordinate data.
In another embodiment, a capacitive sensor is used to confirm or veto the touch data from optical, surface acoustic wave, or force sensors. In this embodiment, when the secondary capacitive sensor is touched by a grounded conductor such as a finger, the resulting current flow indicates a valid touch detection by the primary sensor.
In contrast, a false touch due to either a contaminant (e.g., surface acoustic wave or optical sensors) or a shock or vibration (e.g., force-based touch system) does not result in a coincident current flow in the capacitive secondary sensor, thus invalidating the data from the primary sensor. The secondary capacitive sensor may be a simple discrete sensor such as that provided by a single-contact transparent conductive coating or a more complex capacitive sensor capable of providing touch coordinates for comparison purposes.
In another embodiment of the invention, one in which no touch panel is overlaid on the face of a CRT monitor (e.g., infrared optical system, on-tube surface acoustic wave system, or a non-overlay differential force system), the secondary capacitive sensor is comprised of a resistive coating on the surface of the CRT. A resistive coating of this type may be used to limit charge build-up on the CRT screen. The capacitive sensor of this embodiment utilizes the resistive coating in combination with a current monitoring circuit that measures the amplitude of the electromagnetic, noise signal coupled to the resistive coating. In use, when the screen is touched by a grounded object such as a finger, the detected signal amplitude change exceeds a preset threshold thus indicating a valid touch. In contrast, when an ungrounded object such as a contaminant touches the surface, the detected signal amplitude change does not exceed the preset threshold and the touch is invalidated.
In another embodiment of the invention, a secondary sensor such as a force sensor or a capacitive sensor is used as a means of adding tactile feel to an IR optical sensor based touchscreen. In this embodiment the secondary sensor system is used to determine when the user has made physical contact with the touchscreen. Until actual contact is made, the IR system will not register a touch, thus eliminating false touches that may arise due to the user or an article of the user""s clothing interrupting the IR beam grid. In addition, the secondary sensor system can be used when the system is in a sleep mode, thus eliminating the continuous power drain associated with the IR system.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.